Re: [RFC PATCH] sched/fair: Make tg->load_avg per node

[Dietmar Eggemann]

On 27/03/2023 07:39, Aaron Lu wrote:
> When using sysbench to benchmark Postgres in a single docker instance
> with sysbench's nr_threads set to nr_cpu, it is observed there are times
> update_cfs_group() and update_load_avg() shows noticeable overhead on
> cpus of one node of a 2sockets/112core/224cpu Intel Sapphire Rapids:
> 
>     10.01%     9.86%  [kernel.vmlinux]        [k] update_cfs_group
>      7.84%     7.43%  [kernel.vmlinux]        [k] update_load_avg
> 
> While cpus of the other node normally sees a lower cycle percent:
> 
>      4.46%     4.36%  [kernel.vmlinux]        [k] update_cfs_group
>      4.02%     3.40%  [kernel.vmlinux]        [k] update_load_avg
> 
> Annotate shows the cycles are mostly spent on accessing tg->load_avg
> with update_load_avg() being the write side and update_cfs_group() being
> the read side.
> 
> The reason why only cpus of one node has bigger overhead is: task_group
> is allocated on demand from a slab and whichever cpu happens to do the
> allocation, the allocated tg will be located on that node and accessing
> to tg->load_avg will have a lower cost for cpus on the same node and
> a higer cost for cpus of the remote node.
> 
> Tim Chen told me that PeterZ once mentioned a way to solve a similar
> problem by making a counter per node so do the same for tg->load_avg.
> After this change, the worst number I saw during a 5 minutes run from
> both nodes are:
> 
>      2.77%     2.11%  [kernel.vmlinux]        [k] update_load_avg
>      2.72%     2.59%  [kernel.vmlinux]        [k] update_cfs_group
> 
> Another observation of this workload is: it has a lot of wakeup time
> task migrations and that is the reason why update_load_avg() and
> update_cfs_group() shows noticeable cost. Running this workload in N
> instances setup where N >= 2 with sysbench's nr_threads set to 1/N nr_cpu,
> task migrations on wake up time are greatly reduced and the overhead from
> the two above mentioned functions also dropped a lot. It's not clear to
> me why running in multiple instances can reduce task migrations on
> wakeup path yet.
> 
> Reported-by: Nitin Tekchandani <nitin.tekchandani@xxxxxxxxx>
> Signed-off-by: Aaron Lu <aaron.lu@xxxxxxxxx>

I'm so far not seeing this issue on my Arm64 server.

$ numactl -H
available: 4 nodes (0-3)
node 0 cpus: 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23
node 1 cpus: 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43
44 45 46 47
node 2 cpus: 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67
68 69 70 71
node 3 cpus: 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91
92 93 94 95
node distances:
node   0   1   2   3
  0:  10  12  20  22
  1:  12  10  22  24
  2:  20  22  10  12
  3:  22  24  12  10

sysbench --table-size=100000 --tables=24 --threads=96 ...
/usr/share/sysbench/oltp_read_write.lua run

perf report | grep kernel | head

9.12%  sysbench  [kernel.vmlinux]        [k] _raw_spin_unlock_irqrestore
5.26%  sysbench  [kernel.vmlinux]        [k] finish_task_switch
1.56%  sysbench  [kernel.vmlinux]        [k] __do_softirq
1.22%  sysbench  [kernel.vmlinux]        [k] arch_local_irq_restore
1.12%  sysbench  [kernel.vmlinux]        [k] __arch_copy_to_user
1.12%  sysbench  [kernel.vmlinux]        [k] el0_svc_common.constprop.1
0.95%  sysbench  [kernel.vmlinux]        [k] __fget_light
0.94%  sysbench  [kernel.vmlinux]        [k] rwsem_spin_on_owner
0.85%  sysbench  [kernel.vmlinux]        [k] tcp_ack
0.56%  sysbench  [kernel.vmlinux]        [k] do_sys_poll

Is your postgres/sysbench running in a cgroup with cpu controller
attached? Mine isn't.

Maybe I'm doing something else differently?